The invention pertains to a method for manufacturing of a highly absorbent, polysaccharide based absorption material, wherein a water-containing solution comprising a starting material in the form of a crosslinkable polysaccharide based polymer is subjected to crosslinking in order to obtain a water-swelled gel.
For many applications, such as in absorbent articles intended for absorption of body fluids, it has become increasingly more common to use what is known as superabsorbent materials. Superabsorbent materials are polymers which are capable of absorbing liquid in amounts corresponding to several times of the weight of the polymer and which upon absorption form a water-containing gel.
The main advantage of using superabsorbent materials in absorbent articles is that the volume of the absorbent articles can be considerably reduced when compared to the volume of absorbent articles mainly formed from absorbent fibrous materials such as fluffed cellulose pulp, or the like. Another advantage is that superabsorbents, when compared to fibrous absorbents such as, for instance, fluffed cellulose pulp, have a higher capability of retaining liquid under pressure. Such a property is, for instance, advantageous when the absorption material is used in diapers, incontinence guards or sanitary napkins, since absorbed body fluid is retained in the absorbent article and is not squeezed out of the article, for instance when the user is sitting.
However, a disadvantage with many of the superabsorbent materials presently being used in absorbent articles such as diapers, incontinence protectors or sanitary napkins, is that they are not produced from renewable raw materials. In order to solve this problem, it has been suggested that superabsorbents based on different types of renewable starting materials, such as polysaccharides and, in particular, starch, be used. Unfortunately, the polysaccharide-based superabsorbents which have so far been produced exhibit considerably lower absorption capacity than the commonly used polyacry-late-based superabsorbents. Further, the ability of the polysaccharide-based superabsorbents to retain absorbed liquid when the superabsorbent is subjected to load is low in comparison with polyacrylate-based superabsorbents.
In WO 95/31500 a method for producing absorbent, preferably superabsorbent, foam materials by phase separation and crosslinking of a polymer solution is described. The absorbent materials thus obtained exist in the form of a crosslinked open-celled polymer foam, which implies that fluid may pass between cells. By means of the described production method, it is also said to be possible to obtain biodegradeable absorbent foam materials. Preferred polymers for producing the absorbent materials which are disclosed in WO 95/31500 are hydroxyethyl cellulose (HEC) and hydroxypropyl cellulose (HPC), which are preferably crosslinked with divinyl sulphone (DVS).
The known absorbent foam materials are relatively expensive to produce and are primarily intended for medical applications, such as controlled release systems or as artificial skin and blood vessels. However, a further possible use for the described foam materials is said to be in reusable diapers or the like. The high production cost does, however, mean that the known foam materials would, in practice, not be contemplated as absorption material for absorbent articles intended for single use only.
For these reasons, there exists a demand for an improved superabsorbent material based on renewable raw materials. Accordingly, the absorption capacity for polysaccharide-based superabsorbents needs to be improved in order to make such superabsorbents an equal alternative with regard to absorbency and when compared to the superabsorbents which are commonly being used today. Moreover, there exists a need for a superabsorbent material for use in disposable absorbent articles and which is produced from cheap and readily available renewable starting materials.
The present invention provides a process for the production of superabsorbent materials of the kind mentioned in the introduction and which exhibit improved absorbency as compared to previously known superabsorbent materials of the same type.
The process according to the invention is primarily distinguished in that drying of a crosslinked liquid-swollen gel is carried out by extraction with a polar solvent.
A wide range of solvents may be used for the initial solution, containing the polysaccharide-based polymer starting material. However, the solution containing the starting material is preferably an aqueous solution.
Surprisingly, it has been shown that by drying a crosslinked polysaccharide with a polar solvent, such as ethanol, acetone or isopropanol, a superabsorbent material can be obtained exhibiting superior absorbency when compared to a material of the same composition but dried using another method. The improved absorbency is evident both in a higher absorption capacity and in a greater ability to retain absorbed liquid even when the absorption material is subjected to pressure. The absorbency of a superabsorbent material which has been dried with a polar solvent is considerably higher than that of a corresponding superabsorbent material which has been dried using any other method, regardless of whether the absorbed liquid is water or a salt solution such as urine.
When comparing electron scanning micrographs of crosslinked superabsorbent gels with the same composition but dried in different ways, it is clearly evident that the microstructure of the dried gels, or xero-gels, show significant differences depending on the method of desiccation. Accordingly, an air-dried gel exhibits a dense, compact structure while a gel which has been dried by solvent extraction exhibits a structure with a high degree of microporosity. Vacuum drying produces a structure exhibiting some degree of microporosity and can be said to represent a form between the structure obtained by air-drying and the structure obtained by the solvent drying in accordance with the invention.
A probable explanation of the advantageous effect of solvent drying, is that a commonly occurring phenomenon producing a dense, horny, non-absorbing structure, is avoided. This phenomenon is well known to the person skilled in the art, even though its exact mechanisms have not yet been fully understood. However, the effect is that the crosslinked gel exhibits reduced swelling capability and, thus, reduced absorption capacity. Accordingly, in comparison with conventionally dried gels, a gel which has been dried with a polar solvent exhibits a more open and flexible structure, something that affects the absorption process in a positive way.
The solvent-dried superabsorbent polymer exists in the form of a microporous gel. The superior absorption properties exhibited by the gel are believed to be the result of liquid partly being bound in the gel in a conventional manner and partly being absorbed in the microvoids in the gel. When the gel absorbs liquid, the gel swells, whereby the size of the microvoids increases and the absorption capacity of the gel is enhanced.
The starting material may comprise a polymer blend comprising an electrically charged polysaccharide-based polymer and an electrically uncharged polysaccharide-based polymer. The ratio between the charged polymer and the uncharged polymer is preferably between about 2:1 and about 4:1 and most preferably about 3:1.
A major advantage afforded by the invention is that carboxymethyl cellulose (CMC) can be used as a starting material for the production of a superabsorbent material having high absorption capacity and good liquid retention. The fact that CMC is produced from wood which is a renewable material source and, further, that it is readily available and comparatively low in cost, makes CMC particularly suitable for use in disposable absorbent articles. Moreover, with regard to biodegradability and compostability, CMC exhibits excellent characteristics.
However, it has been found to be less suitable to use CMC as sole starting material for the production of a superabsorbent material, since CMC tends to form intramolecular crosslinks instead of crosslinks between different molecules. An absorption material having particularly good properties may, however, be obtained with a starting material comprising a mixture of CMC in the form of its sodium salt (CMCNa) and hydroxyethyl cellulose (HEC). A suitable proportion between the amount of CMCNa and HEC has thereby been found to be between about 2:1 and about 4:1 and preferably about 3:1. At a lower concentration of HEC, the resulting cross-linked gel does not exhibit sufficient gel strength. High concentrations of HEC should be avoided since the swelling capacity and, accordingly, the absorption capacity will be insufficient if the HEC concentration is too high.
Alternatively, it is possible to use combinations of other charged and uncharged polysaccharides. Some further examples of suitable charged polysaccharides are carboxymethyl starch, oxidized starch and oxidized cellulose. Suitable uncharged polysaccharides include, but are not limited to: ethylhydroxyethyl cellulose (EHEC), hydroxypropyl cellulose (HPC) and hydroxypropyl starch (HPS).
It is further possible to use pectin as starting material.
The polysaccharides are preferably crosslinked with a crosslinking agent producing covalent crosslinks. Some examples of crosslinking agents of this kind are divinylsulphone (DVS), acetaldehyde, formaldehyde, glutaraldehyde, diglycidyl ether, diisocyanates, dimethyl urea, epichlorohydrin, oxalic acid, phosphoryl chloride, trimetaphosphate, trimethylomelamine, polyacrolein. Naturally, it is also possible to use ionic crosslinking or physical crosslinking such as hydrophobic/hydrophilic interactions.